Wave power plants utilise a wave energy converter to convert the rising and falling motion of sea waves into a reversing bi-directional air flow. Oscillating-water-column (OWC) wave power plants, in which the rising and falling motion of the wave surface in an air compression chamber produces a reversing bi-directional air flow, are widely used. Wave power plants, whether of the OWC type or otherwise, include a power take-off device which utilises the reversing bi-directional air flow to generate electrical power. The power take-off device is typically an air turbine which must rotate continuously in the same direction regardless of the air flow direction through the turbine.
The Wells turbine is commonly used in wave power plants and in particular OWC wave power plants. The turbine rotor blades of the Wells turbine comprise symmetrical aerofoils to maintain rotation of the rotor in the same direction irrespective of the air flow direction. The Wells turbine is not, however, suited to operation over a large range of flow rates, with high flow rates in particular having a tendency to cause stall. The operating efficiency of the Wells turbine is typically between 50 and 55%.
Another air turbine, known as the Dennis-Auld turbine, utilises variable pitch rotor blades to improve operating efficiency and is specifically designed for use with OWC wave power plants. A mechanical actuation system is used to vary the setting angle of the rotor blades but this increases the maintenance burden and may lead to reduced reliability. A control system is also needed to control the operation of the mechanical actuation system based on real-time measurements of wave profile parameters such as wave height, wave shape and wave duration. However, given the irregular wave profiles that are typically encountered by OWC wave power plants, it can be difficult to correctly identify when a variation in blade setting angle is actually needed.
In another known turbine arrangement comprising an impulse air turbine, the air flow is initially directed through a set of circumferentially spaced guide vanes which in turn direct the air flow onto the turbine rotor blades at a suitable angle and velocity. In order to handle the reversing bi-directional air flow that is encountered in a wave power plant, a set of circumferentially spaced guide vanes is disposed on each side of the turbine rotor.
The setting angle of the circumferentially spaced guide vanes is typically fixed which means that the geometry of the guide vanes cannot be optimised for air flow in both directions through the turbine. The setting angle of the guide vanes on each side of the turbine rotor is typically selected to direct the air flow onto the rotor blades at an optimum angle and velocity. As a consequence, the setting angle of the guide vanes is not optimised to accept the swirling air flow from the downstream side of the turbine rotor and this leads to a large pressure drop across the downstream guide vanes and a consequent reduction in the operational efficiency of the turbine.
A variable radius impulse air turbine for an OWC wave power plant which attempts to address this difficulty is described in WO-A-2008/012530. The turbine has two identical sets of fixed-geometry circumferentially spaced guide vanes located on opposite sides of the turbine rotor and annular flow passages extending from each set of guide vanes to the turbine rotor. In contrast to the topology described above, the guide vanes on each side of the turbine rotor are radially offset from the rotor blades at a greater radius than the rotor blades. The guide vanes upstream of the turbine rotor impart a swirl motion to the air flow which then accelerates as it flows through the upstream annular flow passage towards the smaller diameter turbine rotor. The air flow subsequently has an opportunity to decelerate as it travels through the downstream annular flow passage before passing through the downstream guide vanes. Although the operational efficiency of this impulse air turbine is greater than that of an impulse air turbine having the topology described above, it is significantly more bulky and requires large diameter ducting and pipe work to direct the air flow to and from the guide vanes.
It would, therefore, be desirable to provide an improved impulse air turbine arrangement for use with a reversing bi-directional air flow which overcomes some or all of the difficulties associated with currently available air turbines including those described above.